1. Field of the Invention
This invention relates to ultrasonic diagnostic imaging, and more particularly, to an ultrasonic imaging system that precisely tracks the relative position of an ultrasound imaging transducer as it is traversed across a surface of an imaging subject.
2. Description of Related Art
Ultrasonic imaging techniques are commonly used to produce two-dimensional diagnostic images of internal features of an object, such as a human anatomy. A diagnostic ultrasonic imaging system for medical use forms images of internal tissues of a human body by electrically exciting an acoustic imaging transducer element or an array of acoustic transducer elements to generate short ultrasonic pulses that travel into the body. The ultrasonic pulses produce echoes as they reflect off of body tissues that appear as discontinuities or impedance changes to the propagating ultrasonic pulses. These echoes return to the imaging transducer, and are converted back into electrical signals that are amplified and decoded to produce a cross-sectional image of the tissues. These ultrasonic imaging systems are of significant importance to the medical field by providing physicians with real-time, high resolution images of the internal features of a human anatomy without resort to more invasive exploratory techniques, such as surgery.
The acoustic imaging transducer which radiates the ultrasonic pulses typically comprises a piezoelectric element or matrix of piezoelectric elements. As known in the art, a piezoelectric element deforms upon application of an electrical signal to produce the ultrasonic pulses. In a similar manner, the received echoes cause the piezoelectric element to deform and generate the corresponding electrical signal. The acoustic imaging transducer is often packaged within a portable or handheld device that allows a sonographer substantial freedom to easily manipulate the imaging transducer over a desired area of interest. The imaging transducer can then be electrically connected via a cable to a central control device that generates and processes the electrical signals. In turn, the control device transmits the image information to a real-time viewing device, such as a video display terminal. The image information may also be stored for later viewing of the diagnostic images.
The individual images produced by such ultrasonic imaging systems comprise discrete frames, with each such frame having a field of view defined by the region traversed by the ultrasonic pulses. As the imaging transducer is manipulated along the body surface to obtain tomographic image slices of an adjacent region in the anatomy, each previous image is replaced on the viewing device by a new image. A drawback of such systems is that the discrete frames do not include any coordinates that relate the precise transducer position to the physical region traversed by the imaging transducer. As a result, the sonographer cannot accurately return to a previously imaged position in order to monitor anatomical changes over time.
An image position location system would enable serial temporal monitoring of anatomical features, and would be of substantial benefit to surgical or therapeutic guidance. For example, such a system would provide accurate measurement of changes to size and position of a tumor, yielding critical information regarding the efficacy of a particular course of treatment. The position-located images could be compared to previously obtained ultrasound images, as well as to images obtained using other modalities such as computerized axial tomography (CAT), magnetic resonance imaging (MRI), or radiotherapy portal images. Moreover, the position-located images could be combined with other such images through known image fusion techniques. The compared or combined images could then provide a physician or sonographer with enhanced information concerning the transforming condition of body tissues which may otherwise be overlooked with serious potential consequences for the patient.
Previously, it has been demonstrated that imaging transducer position coordinates could be collected with a compound B-scanner utilizing a transducer mounted on an arm assembly. Either the arm assembly or the transducer element itself can be provided with sensing devices that track the precise position of the transducer. An example of a compound B-scanner utilizing angular sensing devices on an arm assembly is disclosed in U.S. Pat. No. 4,431,007, to Amazeen et al., for REFERENCED REAL-TIME ULTRASONIC IMAGE DISPLAY.
Despite this potential improvement in the art, conventional compound B-scanners are awkward and inflexible to operate due primarily to the relatively bulky mechanical arm assembly. An additional disadvantage of such scanners is that the correlation accuracy of images taken at different times depends upon the precise orientation of the patient relative to the arm assembly. Unless the patient is returned to the identical position with respect to a previous image scan, the position information obtained by the angular sensing devices would not correlate with the previously obtained position information.
Thus, a critical need exists for an ultrasound imaging apparatus that enables images to be position-located to the anatomical surface of the patient. The apparatus should be compatible with modern handheld ultrasonic imaging transducers without encumbering the transducers with position sensing devices that increase the cost, weight and complexity of such imaging systems.